1. Field of the Invention
The present invention relates to a method for the estimation of a propagation channel from its statistics.
It can be applied in the third-generation mobile telephony networks commonly known as UMTS networks.
It can also be applied to multiple-sensor receivers used for functions of metrology, capacity augmentation, or again for radio-monitoring functions.
2. Description of the Prior Art
In a transmission system, especially one using radio waves, a transmitter sends out a signal in a transmission channel to a receiver. The signal that is sent undergoes amplitude and phase fluctuations in the transmission channel. The signal received by the receiver consists of time-shifted and modified copies of the signal sent. The fluctuations of the signal and the shifts generate interference known by those skilled in the art as intersymbol interference. This interference arises especially from the law of modulation used for the transmission and also from multipath propagation in the channel.
The received signal generally results from a large number of reflections in the channel, the different paths taken by the transmitted signal leading to various delays in the receivers. Thus, the impulse response of the channel represents all the fluctuations undergone by the transmitted signal.
The forthcoming arrival of UMTS networks is obliging equipment manufacturers and suppliers to adapt metrology, capacity-augmentation and radio-monitoring tools to this new standard. The performance characteristics of these tools rely partly on the estimation of the effect of the propagation medium (the radio channel) on the signals sent by the different entities of the network (base station, mobile units etc). This operation is known as propagation channel estimation. It can be used especially to counter the effect of the propagation to improve the quality of the signal received (by equalization) or to bring out information on the propagation medium (namely the directions of arrival and the path delays) in order to implement spatial processing operations such as direction-finding,
Modelling
Signal sent
The composition of the UMTS signal sent by the base station to the mobile units (in the downlink) may be modelled for example according to the scheme shown in FIG. 1.
The signal s(t) consists of several frames, each frame having a fixed duration and comprising a given number of slots. For example a 10 ms frame comprises 15 slots.
FIG. 2 gives a diagrammatc view of a modulator for the downlink (from the base station to mobile units).
The Q-PSK (Quadrature Phase Shift keying) symbols, referenced bq(0), . . . , bq(Ns−1), intended for a user q, are first of all multiplied by a power factor μq, with Ns being the number of symbols sent to this user. Each symbol is then modulated by a sequence known as a spreading sequence, referenced cq with a value of ±1, and a size Nq (spreading factor). Thus, the symbol bq(I) is used to form the sequence μqbq(I) cq(0), . . . , μbq(I) Cq(Nq−1), where I is the index of the symbol.
The sequences cq are orthogonal so that:
            ∑              n        =        0                    min        ⁡                  (                                    N                              q                -                                      ⁢                          N              p                                )                      ⁢                  ⁢                            c          q                ⁡                  (          n          )                    ⁢                        c          p                ⁡                  (          n          )                      =                    N        q            ⁢                          ⁢      if      ⁢                          ⁢      p        =    q  
Else it is equal to 0
The sequences thus formed are then multiplied term by term by a sequence of symbols, (±1±i), called a scrambling code s. This sequence is periodic, with a frame period, and is built so as to simulate a random signal.
The resulting signal to be sent is therefore written as follows:
      y    ⁡          (      n      )        =            ∑              q        =        0            Q        ⁢                              ⁢              μ        q            ⁢                        b          q                ⁡                  (                      ⌊                          n                              N                q                                      ⌋                    )                    ⁢                        c          q                ⁡                  (                      n            ⁡                          [                              N                q                            ]                                )                    ⁢              s        ⁡                  (          n          )                    
with s(n) being the scrambling at a given point in time n where └┘ designates the “integer part” operator and [ ] designates the “modulo” operator. This can also be written as: (1)
                                          y            ⁡                          (              n              )                                =                                    ∑                              q                =                0                            Q                        ⁢                                                  ⁢                                          μ                q                            ⁢                                                d                  q                                ⁡                                  (                  n                  )                                                                    ⁢                                  ⁢                  with          :                                    (        1        )                                                      d            q                    ⁡                      (            n            )                          =                                            b              q                        ⁡                          (                              ⌊                                  n                                      N                    q                                                  ⌋                            )                                ⁢                                    c              q                        ⁡                          (                              n                ⁡                                  [                                      N                    q                                    ]                                            )                                ⁢                      s            ⁡                          (              n              )                                                          (        2        )            
This signal goes to a transmission filter and is than broadcast by the base station to the mobile units.
Signal received by a mobile unit
The signal is received on a network of sensors after crossing the radio channel. After sampling, it can take the form:
                              x          ⁡                      (            n            )                          =                                            ∑                              k                =                0                                            L                -                1                                      ⁢                                                  ⁢                                          y                ⁡                                  (                                      n                    -                    k                                    )                                            ⁢                              h                ⁡                                  (                  k                  )                                                              +                      b            ⁡                          (              n              )                                                          (        3        )                            where the bold characters designate vectors, and        L corresponds to the spread of the channel expressed in number of chips, the indices n and k correspond to chips.        x(n) represents:        when the sampling is done at the chip rate, x(n) is the vector of the signals received at the instant n on each sensor,        when over-sampling is done at a rate corresponding to one chip/2, x(n) is the vector of the signals received at the instants n and n+chip/2.        h(k) is the multi-sensor channel, and        b(n) is an additive noise combining the interference phenomena coming from the other base stations and the thermal noise.        
Signal received by a station
FIG. 3 gives a diagrammatic view of one possibility of modelling the uplink (from the mobile units to base station) described in detail here below in the context of the method according to the invention.
Known methods
There are many known methods for estimating the response of a propagation channel.
For example, one classic method proceeds by correlation of the received signal with shifted versions of a known learning sequence of the receiver. Of this sequence, only the steps needed for the understanding of the invention are recalled here below.
For the user 0, for example, the channel impulse response Is estimated, in a classic manner, by correlating several shifted versions of the received signal x(n+k) (one version being shifted by the sample k) by the learning sequence:
                                          h            ^                    ⁡                      (            k            )                          =                              1                                          N                0                            ⁢              P                                ⁢                                    ∑                              n                =                0                                                              N                  0                                ⁢                P                                      ⁢                                                                      ⁢                                                s                  ⁡                                      (                    n                    )                                                  *                            ⁢                                                c                  0                                ⁡                                  (                                      n                    ⁡                                          [                                              N                        0                                            ]                                                        )                                            ⁢                                                                    b                    0                                    ⁡                                      (                                          ⌊                                              n                                                  N                          0                                                                    ⌋                                        )                                                  *                            ⁢                              x                ⁡                                  (                                      n                    +                    k                                    )                                                                                        (        4        )            
where P represents the number of learning symbols, and
N0P the number of learning chips, or the number of pilot chips
By combining the equations (1), (2), (3) and (4), the detailed expression of the estimated channel corresponds to (5):
            h      ^        ⁢          (      k      )        =                    ∑                  i          =          0                L            ⁢                          ⁢                        h          ⁡                      (            l            )                          ⁢                  1                                    N              0                        ⁢            P                          ⁢                              ∑                          n              =              0                                                                        N                  0                                ⁢                P                            -              1                                ⁢                                          ⁢                                                    d                0                            ⁢                              (                                  n                  +                  k                  -                  l                                )                                      ⁢                                          d                0                *                            ⁢                              (                n                )                                                          +                  ∑                  q          =          1                Q            ⁢                          ⁢                        μ          q                ⁢                              ∑                          l              =              0                        L                    ⁢                                          ⁢                                    h              ⁢                              (                l                )                                      ⁢                          1                                                N                  0                                ⁢                P                                      ⁢                                          ∑                                  n                  =                  0                                                                                            N                      0                                        ⁢                    P                                    -                  1                                            ⁢                                                          ⁢                                                                    d                    q                                    ⁢                                      (                                          n                      +                      k                      -                      l                                        )                                                  ⁢                                                      d                    0                    *                                    ⁢                                      (                    n                    )                                                                                            +                  1                              N            0                    ⁢          P                    ⁢                        ∑                      n            =            0                                                              N                0                            ⁢              P                        -            1                          ⁢                                  ⁢                              b            ⁢                          (                              n                +                k                            )                                ⁢                                    d              0              *                        ⁢                          (              n              )                                          
The properties of the sequences dq(n) are such that (6):
                    1                              N            0                    ⁢          P                    ⁢                        ∑                      n            =            0                                                              N                0                            ⁢              P                        -            1                          ⁢                                  ⁢                                            d              0                        ⁢                          (                              n                +                k                -                l                            )                                ⁢                                    d              0              *                        ⁢                          (              n              )                                            =                  1        ⁢                                  ⁢        si        ⁢                                  ⁢        k            =              l        ⁢                                  ⁢                                  =                              1                                                            N                  0                                ⁢                P                                              ⁢                                          ⁢          sinon                                        1                              N            0                    ⁢          P                    ⁢                        ∑                      n            =            0                                                              N                0                            ⁢              P                        -            1                          ⁢                                  ⁢                                            d              q                        ⁡                          (                              n                +                k                -                l                            )                                ⁢                                    d              0              *                        ⁡                          (              n              )                                            =          1                                    N            0                    ⁢          P                    
Thus, the estimate of the channel impulse response is expressed as a function especially of the response of the channel h (k) and of the three terms representing existing interference (7):
            h      ^        ⁢          (      k      )        =            h      ⁢              (        k        )              +                  ∑                              l            =            0                    ,                      i            ≠            k                          L            ⁢                          ⁢                        h          ⁢                      (            l            )                          ⁢                  1                                    N              0                        ⁢            P                          ⁢                              ∑                          n              =              0                                                                        N                  0                                ⁢                P                            -              1                                ⁢                                          ⁢                                                    d                0                            ⁢                              (                                  n                  +                  k                  -                  l                                )                                      ⁢                                          d                0                *                            ⁢                              (                n                )                                                          +                  ∑                  q          =          1                Q            ⁢                          ⁢                        μ          q                ⁢                              ∑                          l              =              0                        L                    ⁢                                          ⁢                                    h              ⁢                              (                l                )                                      ⁢                          1                                                N                  0                                ⁢                P                                      ⁢                                          ∑                                  n                  =                  0                                                                                            N                      0                                        ⁢                    P                                    -                  1                                            ⁢                                                          ⁢                                                                    d                    q                                    ⁢                                      (                                          n                      +                      k                      -                      l                                        )                                                  ⁢                                                      d                    0                    *                                    ⁢                                      (                    n                    )                                                                                            +                  1                              N            0                    ⁢          P                    ⁢                        ∑                      n            =            0                                                              N                0                            ⁢              P                        -            1                          ⁢                                  ⁢                              b            ⁢                          (                              n                +                k                            )                                ⁢                                    d              0              *                        ⁢                          (              n              )                                                          where Q is the number of users of the propagation channel,        l, k are indices corresponding to chips, and        n is the index of the instant considered        
Three types of interference disturbing the estimation are thus identified:                the first term corresponds to the self-correlation for the sequence do,        the second term corresponds to the interference from the other users, and        the third term corresponds to the contribution of the external noise and the thermal noise.        
This technique is efficient when the learning sequence is long and when the propagation channel does not change or undergoes little change in time. In the case of a fast variation of the propagation channel, it becomes necessary to estimate it on fairly short periods of time.
Another technique, known as the “least error squares technique” improves the above method by removing the need for self-correlations of the learning sequence.
The patent FR 2 762 164 discloses a method to estimate the impulse response of the transmission channel. This method uses the estimation of the space-time covariance matrix Γ of the impulse response of the channel. The method considers that the estimation noise is white, with a power B, and independent of the channel. In this case, the matrix Γ is estimated by {circumflex over (Γ)}={circumflex over (Δ)}−Bl with Δ being the matrix of space-time covariance of the estimated channel. If B is unknown, it can be estimated by the smallest eigenvalue of the estimated matrix Δ. It can also be fixed at a threshold value, Such an assumption is perfectly suited to a system in which a propagation channel is dedicated to a user, for example the GSM system. Furthermore, the proposed technique relates to single-sensor receivers.
However, a method of this kind is no longer suited to signals including several users, such as the UMTS signals where the noise Is neither white nor independent of the channel, especially owing to multiple-user interference.
An object of the present invention is a method to estimate the propagation channel from its statistics, which are themselves estimated by expressing especially the noise from the matrix of empirical correlation of the observations.